Wet strength paper



United States Patent WET STRENGTH PAPER Francis A. Bonzagni,Springfield, Mass., assignor to Monsanto Chemical Company, St. Louis,Mo., a corpora= tion of Delaware No Drawing. Filed May 23, 1956, Ser.No. 586,656

7 Claims. (Cl. 162-167) The present invention relates to paper havinggood wet tensile strength and to methods for preparing same. Moreparticularly, the present invention relates to paper having certainnovel resins deposited upon the fibers thereof to enhance its wettensile strength.

It is known that the deposition of certain resins on the fibers of paperwill significantly enhance the wet tensile strength of the paper. Suchresin-impregnated papers are referred to generically in the art as wetstrength papers. Although the presently available wet strength papersconstitute a major technical advance, the art continues to seek papershaving still higher wet tensile strengths.

A limiting feature of the wet strength papers presently available isthat the paper, as it is manufactured and comes off the Fourdriniermachine, has only a small percentage of its ultimate wet tensilestrength. To develop its ultimate wet tensile strength, the paper, afterits manufacture, must be heated to elevated temperatures or aged forrelatively long periods of time at ambient air temperature. Eithersubsequent treatment procedureentails an extra processing step inmanufacture and adds to the cost of the paper.

It is an object of this invention to provide paper having improved wettensile strength and to methods for preparing same.

Another object of this invention is to provide paper having certainnovel resins deposited on the fibers thereof to improve its wet tensilestrength.

Still another object of this invention is to provide paper having a highpercentage of its ultimate wet tensile strength as manufactured andwhich does not require a subsequent heat or aging treatment to develophigh wet tensile strength.

Other objects and advantages of this invention will be come apparentfrom the following detailed description thereof.

It has been discovered that paper having high wet tensile strength canbe prepared by depositing certain novel resins on the paper fibers. Suchresins are water-dispersible, acid-aged, cationic, sulfite-modified,melamineformaldehyde resins which, before acid aging, contains 2.5-4.5mols of formaldehyde and up to 0.2 mol of sulfonate groups per mol ofmelamine and have a viscosity of at least about 300 centipoises whenmeasured in a 55 aqueous solution at 25 C. A very desirable feature ofthe wet strength papers of this invention is that they have a highpercentage of their ultimate wet tensile strength as manufactured on theFourdrinier machine and do not require post manufacture heating or longaging to develop high wet tensile strength.

The following examples are set forth to illustrate more clearly theprinciple and practice of this invention to those skilled in the art.All parts are by weight.

EXAMPLE I Part A A sulfite-modified, melamine-formaldehyde resin is pre-"ice pared by reacting together 1 molar proportion of melamine, 3.6molar proportions of formaldehyde (charged as a 37% formalin solution)and 0.17 molar proportion of sodium bisulfite. After mixing thereactants together and adjusting the pH of the solution to 9.5 withsodium hydroxide, the reaction mixture is heated for five hours at atemperature of 95100 C. The pH of the reaction mixture is followed andsodium hydroxide is added throughout the reaction period as required tomaintain the pH between 9.0 and 9.5. The resin is obtained as a 55%aqueous solution which has a viscosity of 1500 centipoises at 25 C. Theresin is recovered as a dry powder by spray-drying.

Part B diluted to 3% resin solids and does not undergo any measurablechange in physical properties when stored for periods of up to at least30 days. In the subsequent Examples III-XI, the 3% colloidal resinsolution is permitted to stand for 16 hours at room temperature beforeuse.

EXAMPLE H Part A An unmodified melamine-formaldehyde resin is preparedas a control resin to be employed in comparing the wet tensile strengthimparting properties of the novel sulfite-modified melamine-formaldehyderesins employed in the practice of this invention. This control resin isprepared by reacting 1 molar proportion of melamine with 3.6 molarproportions of formaldehyde at -100 C. at a pH of 9.0-9.5. The resin ispolymerized to the extent that a 60% aqueous solution thereof separatesinto two phases when cooled to 20 C. The resin is recovered by spraydrying and corresponds in properties to those of the commerciallyavailable melamine-formaldehyde resins that are employed in themanufacture of wet strength paper.

Part B A cationic colloid of the resin of Part A above is prepared asdescribed in Example I, Part B, except that only 4.8 parts of 37%hydrochloric acid are employed. The lower quantity of acid is employedas maximum resin properties are obtained therewith. The colloidalsolution has a blue haze and the resin particles carry a positivecharge. The colloidal resin solution is stable for at least 30 days atroom temperature. The colloidal resin solution is permitted to stand for16 hours at room temperature before use in the subsequent examples.

-In several of the subsequent examples the physical properties of papersheets are reported. In all cases, the sheets are prepared on a Nobleand Wood sheet machine with the paper sheets measuring 8 inches by 8inches and weighing 2.5 grams. The sheets are dried for 6 minutes atZOO-205 F. to approximate the heat history of paper prepared oncommercial Fourdrinier machines. Tensile data are obtained on a Schoppertensile tester and the results are reported on the basis of pounds perinch width unless otherwise noted. The sheets on which wet tensilestrengths are determined are wetted by thoroughly soaking the papersheet in distilled water and removing the excess water by pressing thesheets tightly between large blotters.

EXAMPLE III The acid colloid of the sulfite-modified, me1amine- 3 4formaldehyde resin prepared in Example I, Part B, is TABLE IV evaluatedat several levels of resin concentration by incorporating the resin inan unbleached northern kraft Resin ,32% ggf pulp furnish. Two parts ofalum are included in the Resin On lbs/inch furnish and the pH thereof isadjusted to 4.5 before 5 Paper preparing the paper sheets. For purposes(g comparisgn, 1 7 similar papers are prepared from northern aft furnises 1 containing the control, unmodified melamine-formaldesulfitedMelamme 2-; hyde resin colloid prepared in Example II, Part B. The 5 3levels of resin concentration employed and the wet tensile gfigqlqfqi gggjifj fff jfifiifj i1 1 strength of the resulting sheets are shown inTable I. 1

TABLE I As in Examples III, IV and V, markedly higher Wet Wt. Wettensile values are obtained with the sulfite-modified resin. Resin155351 631 i sifii c a EXAMPLEVII Paper Several sulfite-modified,melamine-formaldehyde resins of varying degrees of polymerization areprepared to illusgg trate the eiiect of resin molecular weight (measuredby Sulfited MelamineResin-Expl. I, Part B 115 413 0 viscosity) on thewet strength generating properties of 2:3 $18 the acid-aged colloidprepared therefrom. To this end, 0.5 1.4 Example I, Part A, is repeatedexcept that the reaction cilltggtbgwsulfitedMelammeResin-31x91 mixtureis heated at 95l00 C. until the resin has a 2:5 3:7 viscosity of 7000centipoises as measured in a 55% aqueous solution at 25 C. Aliquots ofthe resin are Withdrawn when the viscosities of 55% aqueous solutions Ateach level of resin concentration materially higher wet thereof reach500 1000 500 and 7000 centipoises, tBHFiIe Strengths are obtained withsulfite-modified and these aliquots are converted into acid-agedcolloids Tesmas described in Example I, Part B. These resin colloidsEXAMPLE IV are incorporated in a northern kraft pulp furnish at a levelExample III is repeated except that the resins are inof 15% resin basedon the y Weight, of i P p corporated in a bleached sulfite pulp furnishcontaining fibers The tenslle strengths of the fesultmg Sheets 5% alum.The concentrations, of resin employed and the are Set forth m Table wettensile strength of the resulting sheets are set forth TABLE V inTableH'a v t o t 1 1 WtTn'l TABLE II 65111 150051 3 en 1P0 S95 {abs/5105111 e,

Wt. Wet 500 4. 7 Resin Percent Tensile, 1,00 4. 8 Resin 0n lbs/inch 41,500 50 Paper 7,000 5. l

5 1. 7 1 Determine in 55% aqueous solution at 25 C. Sulfited MelamineResin-Expl. I; Part ]3 {g It is seen from the above Table V that the wettensile 2.5 3.8 strength of the paper increases with: the viscosity ofthe Control-Nousulfited Melamine Resin-Ex- 9'3 sulfitemodified,melamine-formaldehyde resin fronrwhich ample HPEMB 1:5 115 the acid-agedcolloid is prepared; Other data indicate 0 5 thatthe wet tensilestrength proper-ties ofthetpaper fall off rather rapidly when theviscosity of the sulfite-modified, Matellally l wetfenslle strengths areObtalned Wlth melamine-formaldehyde resin isless' than about 300centithe sulfite'modlfied poises when measured in a. aqueous solution atEXAMPLE V 250 C. EXAMPLE v 11 Example III is repeated except that theresins-are in- F h L i corporated in a high alpha cellulose pulpfurnish. The our resms modified Varymg amounts of sodmfn results are setforthin Table EL 55 sulfite are prepared to illustrate the effect oftheresin sulfonate group content upon thewettensile strength TABLE IIIproperties ofpaper prepared therefrom. Thefour resins are prepared bythe procedure described in Example I, Resin iggs t Part A, except thatthe-molar proportion of sodium Resin on lbslincfi sufiteper mol ofmelamme included 1n the reaction mix- Paper ture is ad usted to,respectively, 0.12, 0.14, 0.17- and 0.19. Acid-aged colloids of theresins are prepared as 1.0 1.0 described in Example I, Part B. Theresulting acid-aged Sulfite elami e D -L 3:3 1;: resin colloids are.incorporated at the 1.5% level in a -0 high alpha cellulose pulpfurnish. Thewet tensile Control-Nonsulfited Melamine Res1nEx- 3 8:?strengths of the resulting sheets areset forth in Table ample II, Part B3. 0 O 7 VI.

TABLE VI- As in Examples III and IV, materially superior results M015sulfit lM l hl l m Wet Tensile are obtained with the sulfite-modifiedresin. lbs/inch EXAMPLE VI 0 12 z 1 Example III is repeated except thatthe resin is incorpo- 8i; rated in a bleached southern kraft pulpfurnish. The 1.6 results are set forth in Table IV.

As seen in the above Table VI, the wet tensile strength properties ofthe -paper fall off markedly when the sulfonate group content of theresin colloid approaches 0.2 mol of sulfonate groups per mol ofmelamine.

EXAMPLE IX The quantity of acid employed in preparing the acidagedcolloid of the sulfite-modified, melamine-formaldehyde resin has only asmall effect upon the wet tensile strength properties of the paperprepared therefrom as illustrated by this example. Three acid-agedcolloids of the resin of Example I, Part A, are prepared as described inExample I, Part B, except that the quantity of 37% hydrochloric acidemployed is adjusted to 4.7 parts, 5.9 5

TABLE VII Amount H01 EmployedParts HOl/100 parts resin Wet Tensile,

lbs/inch EXAMPLE X Papers prepared with the cationic resin colloids ofthis invention develop a greater percentage of their ultimate wettensile strength as manufactured, than do corresponding papers preparedwith cationic colloids of conventional melamine-formaldehyde resins. Toillustrate this fact, paper is prepared from 4 different pnlps, highalpha cellulose pulp, bleached southern kraft pulp, bleached sulfitepulp and unbleachednorthern kraft pulp. The cationic resin colloid ofExample I, Part B, is incorporated into the pulp furnish at severaldifierent resin levels, and sheets are prepared therefrom by thestandard procedure previously described. For comparison purposes,similar papers are prepared employing the cationic resin colloid of theunmodified melamine-formaldehyde resin of Example II, Part B. Inaddition, papers prepared by the previously described standard procedureare aged for 30 minutes at 220 F. to further cure the wet strengthresins 20 and develop the optimum wet tensile strength of the papers.

Details as to the composition of the papers tested, the

uncured and cured wet tensile strengths thereof and the percentages ofthe ultimate wet tensile strength obtained in the paper as manufactured,i.e., after drying for 6 minutes at 200 F., are set forth in Table VIII.

From Table VIII it will be noted that with each pulp type studied, andat each resin level in said pulp, paper prepared with the cationic,sulfite-modified, melamine-formaldehyde resin colloid develops a higherTABLE VIII Wt. Per- Uncured Cured Type Pulp Resin cent Wet Ten- Wet'Ien- Tu/TcXiOO 3 Resin sile sile On Paper Percent 1.0 1.0 3.8 26Sulfited Melamine Resin-Expl. I, Part B 2.0 1. 4 4. 9 28 3.0 1. 9 6. 131 4.0 2. 2 6.9 32

Ave.= High Alpha Cellulose Control-Non-sulflted Melamine Resin- 1. 00.60 3. 2 19 Expl. II, Part B 2.0 0. 70 4. 4 16 3.0 0. 70 4. 6 15 4. 00.94 6. 2 15 Ave.=16

1.0 2. 4 3. 7 65 Sulfited Melamine Resin-Expl. I, Part B.-.- l. 5 2. 94. 6 64 2. 5 4. 1 5.9 70

Ave.=66 Bleached Southern Kraft Control-Non-sulfited Melamine Resin 1.0 1. 8 3. 3 55 Expl. II, Part B 1. 5 2.3 4. 3 54 2. 6 2. 8 5. 6

Ave-53 1.0 2.4 3 l 78 Sulfited Melamine Resin-Expl. I, Part B 1.5 2. 8 38 74 2.5 3.8 5 0 76 Ave.=76 Bleached Sulfite Control-Non-sulfitedMelamine Resin- 1. 0 1.2 2. 2 55 Expl. I, Part B 1. 5 1. 5 2. 7 56 2.5 1. 5 3. 3 46 Ave.=52

0. 5 2. 1 3. 5 60 Sulfited Melamine Resin-Expl. I, Part B.... 1.0 3. 25. 2 62 1.5 4.3 6.5 66 2. 5 5.0 7. 5 67 Ave.=64 Unbleached NorthernKraft Contro1Non-sulfited Melamine Resin 0.5 1. 5 3.1 48 Expl. II, Part:B 1.0 2. 3 5.0 46 1.5 3.0 5.9 51 2. 5 3. 6 7.5

Ave.=48

Uncured wet tensile=sheet dried 6 minutes @200 F. Cured wettensile=sheet dried 6 minutes 200 F. and heated 30 minutes 220 F,

a TulTc=Uncured wet tensile/cured wet tensile.

percentage of its ultimate wet tensile strength as manufactured, i.e.,when dried only 6 minutes at 200 F., than does the control.

EXAMPLE X1 A further outstanding characteristic of the papers of thisinvention is that they have high brightness values, i.e. they reflect ahigh percentage of incident light and appear very bright. To illustratethis feature of the invention, 4 batches of paper are prepared employingthe acid colloid of a sul-fited melamine resin containing 0.14 mol ofsulfonate groups per mol of melamine. The resin colloid employed isdescribed in Example VIII. The resin colloid is incorporated in a highalpha cellulose pulp furnish containing 2% alum in amounts suflicient toconstitute, respectively, 2.0%, 3.0%, 4.0% and 5.0% of the paper-makingfibers. For purposes of comparison, 4 similar batches of paper areprepared employing the acid colloid of the nonsulfited melamine resindescribed in Example II, Part B. The color of the two resin colloidsolutions incorporated in the paper furnishes are substantiallyidentical. The brightness values of the two sets of paper are determinedby TAPPI Method 452 m- 48 employing a Wratten #49 blue filter. Theresults are set forth in Table IX.

TABLE IX Resin Reflec- Resln Level tance,

percent 38 ii Sulfited Melamine Resin 73 5.0 74 2.0 70 ControlNon-sulfited Melamine ResinExpl. 3.0 69 II, Part B. 2.0 6g

From the above table it is seen that paper prepared with the sulfitedmelamine resin is appreciably brighter than paper prepared with thecontrol nonsulfited melamine resin at each resin level tested. Inaddition, the brightness of the papers of this invention remainsconstant or increases as the concentration of the resin incorporated inthe paper is increased, whereas the brightness of the papers preparedfrom the prior art nonsulfited melamine resin decreases as theconcentration of resin incorporated in the paper is increased.

The resins employed in the preparation of the high wet strength papersof this invention are water-dispersible, acid-aged, cationic,sulfite-modified, melamine-formaldehyde resins having critical molecularproportions and degrees of polymerization as subsequently set forth ingreater detail. The term water-dispersible resin" is used in a genericsense to cover truly water-soluble resins as well as colloidallydispersible resins.

The sulfite-modified, melamine-formaldehyde resins from which theacid-aged, cationic resins are prepared contain for each mol of melamine2.5-4.5 mols and preferably 3.0-4.0 mols of formaldehyde, with optimumproperties being obtained in resins containing 3.6 mols of formaldehyde.Similarly, the resins should contain up to 0.2 mol and preferably0.10-0.18 mol of sulfonate groups per mol of melamine, with optimumproperties being obtained in resins containing 0.12-0.15 mol ofsulfonate groups per mol of melamine. The resins should never containmore than about 0.2 mol of sulfonate groups per mol of melamine as theresulting resins coagulate and precipitate when placed in contact withstrong acids and as a result are inoperable in the practice of thisinvention.

The sulfite-modified, melamine-formaldehyde resins should have a highdegree of polymerization before they are converted into cationiccolloids by acid aging. The degree of polymerization can be measuredconveniently by the viscosity of the resin in aqueous solution. As

set forth in Example VII, resins prepared under otherwise identicalconditions impart higher wet strength to paper as the viscosity of theresin is increased. To be suitable for use in this invention, thesulfite-modified, melamine-formaldehyde resins should, before beingconverted into a cationic colloid by acid aging, have a viscosity of atleast about 300 centipoises when measured in a 55% solution at 25 C. Theonly upper limit on the viscosity of the resin is the consideration thatit must dissolve in the strong acid solution employed to convert thatresin into a water-dispersible, cationic colloid.

The sulfite-modified, melamine-formaldehyde resins are convenientlyprepared by inter-reacting melamine, formaldehyde and an alkali metalbisulfite or a progenitor thereof in an aqueous solution. Preferably,such reactions are carried out in alkaline solutions, e.g. at a pH of8.5 or higher.

The sulfite-modified, melamine-formaldehyde resins are converted intocationic colloids by treating said resins with strong acid solutions.For the development of optimum properties, the sulfite-modified,melamine-formaldehyde resin is dissolved or dispersed in water and theacid is added thereto. Alternatively, the resin can be added to a strongacid solution.

The quantity of acid employed in preparing the cationic resin colloidsis sufficient to adjust the pH of the resin solution to the range of0.5-3.5 and preferably 1.5-2.5. It is preferred to employ hydrochloricacid in the preparation of the cationic resin, but other strong acidssuch as phosphoric acid, formic acid, etc. also may be em ployed ifdesired.

The efiiciency of the cationic resins in enhancing the wet tensilestrength of paper is atfected by the length of time that the resin isaged in the acid solution. When the resin solution is dilute, i.e.contains less than 5% resin solids, 5-15 hours at room temperature areusually required for the development of optimum efiiciency and littleimprovement is obtained by aging the dilute resin solutions for longerperiods. Such dilute resin solutions are very stable and can be storedfor up to 30 days at room temperature Without adversely aiiecting theproperties and efliciency of the cationic resin. In more concentratedresin solutions, i.e. those containing more than 10% resin solids, theefficiency of the resin develops more rapidly, usually within 3-5 hoursat room temperature. Such concentrated resin solutions are relativelyunstable, however, and after the cationic resin develops its maximumefliciency, on further aging, it rapidly loses its efficiency andeventually forms a non-water dispersible precipitate. Such precipitatesare usually formed after about 7 hours at room temperature. In apreferred procedure, the resin-is aged about 3 hours at room temperaturein a concentrated solution, e.g. at 10-15% resin solids, and thereafteris diluted to less than 5% resin solids and stored until used.

The cationic resin colloids employed in the practice of the presentinvention are claimed and more fully described in my copendingapplication Serial Number 586,655, filed of even date herewith, now US.2,863,842, and that description is incorporated herein by reference.

The wet strength papers of this invention can be prepared by applyingthe cationic, sulfite-modified, melamineformaldehyde resin to the paperfibers in any known manner. The most practical method for preparing thewet strength paper is to add a small quantity of the cationic resin tothe paper furnish at any point prior to the preparation of the sheettherefrom. If desired, however, the cationic resin may be applied to thepaper fibers in other known manners, e.-g. as by impregnatingsemifinshed paper sheets with a solution of the cationic resin colloid.The amount of cationic resin applied to the paper is that customarilyemployed with other wet strength resins, viz., from about 0.1% to about5.0% by weight based on the dry paper.

The wet strength papers of this invention have three very importantcharacteristics which distinguish them from wet strength papers preparedfrom previously known melamine-formaldehyde resins. First, the papers ofthis invention have higher ultimate wet tensile strengths than wetstrength papers hereto available. Secondly, the papers of this inventionhave a higher percentage oftheir ultimate wet tensile strength asmanufactured than do wet strength papers of the prior art. Thecombination of these two factors leads to wet tensile strength values inthe paper as manufactured that are frequently 25- 35% higher than thevalues that can be obtained in similarly prepared paper employing thewet strength resins previously available to the art. Thirdly, the papersof this invention have appreciably higher brightness values, i.e. appearwhiter, than do paper manufactured from previously knownmelamine-formaldehyde resins, cf. Example XI. The differences inbrightness values that can be obtained, i.e. up to 6 units, areconsidered to be highly significant in the paper art.

The above descriptions and particularly the examples are set forth byway of illustration only. Many other variations and modificationsthereof will be obvious to those skilled in the art and can be madewithout departing from the spirit and scope of the invention hereindescribed.

What is claimed is:

1. A process for preparing paper having high wet tensile strength whichcomprises adding to an aqueous suspension of cellulosic paper-makingfibers a water-dispersible, acid-aged, cationic, sulfite-modified,melamineformaldehyde resin and forming the treated cellulosic fibersinto a waterlaid sheet, the quantity of resin added to the suspension ofcellulosic paper-making fibers being sufiicient to enhance the wettensile strength of the waterlaid sheet; said water-dispersible,acid-aged, cationic, sulfite-modified, melamine-formaldehyde resinhaving been sewn I 10 prepared by dispersing and aging asulfite-modified, mel amine-formaldehyde resin in an aqueous solutionhaving a pH of 0.5-3.5, said sulfite-modified, melamine-formaldehyderesin, before acid-aging, containing 2.5-4.5 mols of formaldehyde and upto 0.2 mol of sulfonate groups per mol of melamine and having aviscosity of at least about 300 centipoises when measured in a aqueoussolution at 25 C.

2. A process as in claim 1 wherein said aqueous solution has a pH of1.5-2.5.

3. A process as in claim 1 wherein said aqueous solution is an aqueoussolution of hydrochloric acid.

4. A paper sheet prepared by the method of claim 1.

5. A process as defined in claim 1 wherein the quantity of resin addedto the aqueous suspension of cellulosic paper-making fiber constitutes01-50% of the dry weight of the paper-making fibers.

6. A process as in claim 5 wherein the sulfite-modified,melamine-formaldehyde resin, before acid-aging, contains 3.0-4.0 mols offormaldehyde and 0.10-0.18 mol of sulfonate groups per mol of melamine.

7. A process as defined in claim 5 wherein the sulfitemodified,melamine-formaldehyde resin, before acidaging, contains about 3.6 molsof formaldehyde and approximately 0.12-0.15 mol of sulfonate groups permol of melamine.

References Cited in the file of this patent UNITED STATES PATENTS2,047,599 Auten Sept. 10, 1946 2,559,220 Maxwell July 3, 1951 FOREIGNPATENTS 611,244 Great Britain Oct. 27, 1948 636,892 Great Britain May10, 1950 654,305 Great Britain June 13, 1951 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTIDN Patent No, 2,94%396 Francis. A. Bonzagni It ishereby certified that error appears in the printed specification of theabove numbered patent requiring correction and that the said LettersPatent should read as corrected below.

Column 1OY line 30 list of references cited, under "UNITED STATESPATENTS? for "2,047 599" read 2,40%599 e Signed and sealed this 11th dayof April 1961.

(SEAL) Attest:

ERNEST W1 SWDER ARTHUR W. CROCKER Attesting Oflicer A ti Commissioner ofPatents August 16 1960.

1. A PROCESS FOR PREPARING PAPER HAVING HIGH WET TENSILE STRENGTH WHICH COMPRISES ADDING TO AN AQUEOUS SUSPENSION OF CELLULOSIC PAPER-MAKING FIBERS A WATER-DISPERSIBLE, ACID-AGED, CATIONIC, SULFITE-MODIFIED, MELAMINEFORMALDEHYDE RESIN AND FORMING THE TREATED CELLULOSIC FIBERS INTO A WATERLAID SHEET, THE QUANTITY OF RESIN ADDED TO THE SUSPENSION OF CELLULOSIC PAPER-MAKING FIBERS BEING SUFFICIENT TO ENHANCE THE WET TENSILE STRENGTH OF THE WATERLAID SHEET, SAID WATER-DISPERSIBLE, ACID-AGED, CATIONIC, SULFITE-MODIFIED, MELAMINE-FORMALDEHYDE RESIN HAVING BEEN PREPARED BY DISPERSING AND AGING A SULFITE-MODIFIED, MELAMINE-FORMALDEHYDE RESIN IN AN AQUEOUS SOLUTION HAVING A PH OF 0.5-3.5, SAID SULFITE-MODIFIED, MELAMINE-FORMALDEHYDE RESIN, BEFORE ACID-AGING, CONTAINING 2.5-4.5 MOLS OF FORMALDEHYDE AND UP TO 0.2 MOL OF SULFONATE GROUPS PER MOL OF MELAMINE AND HAVING A VISCOSITY OF AT LEAST ABOUT 300 CENTIPOISES WHEN MEASURED IN A 55% AQUEOUS SOLUTION AT 25*C. 